EP4158180A1

EP4158180A1 - A direct injection gaseous fuel feeding system for a two-stroke internal combustion piston engine, a two-stroke internal combustion piston engine and method of operating a two-stroke internal combustion piston engine

Info

Publication number: EP4158180A1
Application number: EP20728707.9A
Authority: EP
Inventors: Yao XIAO; Mikko Mäenpää; Martin RÖNNBERG; David C. Jay; Antti VUOHIJOKI
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2020-05-25
Filing date: 2020-05-25
Publication date: 2023-04-05
Also published as: KR20230003221A; CN115667694A; CN115667694B; WO2021239204A1; KR102807206B1

Abstract

Invention relates to a direct injection gaseous fuel feeding system (10) for an internal combustion piston engine (1), the fuel feeding system (10) comprising a liquefied gas storage tank (14) configured to store liquefied gas in cryogenic conditions, a fuel feed line (16) extending from the tank to a fuel injector in the engine (1), which fuel feed line (16) comprises at least the following: a liquefied gas high pressure pump unit (18), a heat exchanger unit (22) for evaporating the liquefied gas and heating the gaseous gas, and a gaseous gas accumulator (24) between the heat exchanger unit (22) and the fuel injector, and at least two gaseous gas fuel injectors in fluid communication with the gaseous gas accumulator (24). The liquefied gas high pressure pump unit (18) comprises a reciprocating piston having a pumping part (32) and a drive part (34), and a hydraulic drive assembly (36) arranged to controllably subject hydraulic power fluid from a source of hydraulic power fluid (102) to the drive part (34) of the reciprocating piston for driving the piston in reciprocating manner.

Description

A direct injection gaseous fuel feeding system for a two-stroke internal combustion piston engine, a two-stroke internal combustion piston engine and method of operating a two-stroke internal combustion piston engine Technical field

[001] The present invention relates to a direct injection gaseous fuel feeding system for a two-stroke internal combustion piston engine according to the pre amble of claim 1. Invention relates also to a two-stroke internal combustion piston engine provided with a direct injection gaseous fuel feeding system and method of operating a two-stroke internal combustion engine.

Background art

[002] Internal combustion piston engine is widely utilized in providing mechan ical power in land based power plant and in marine vessel for producing electric power and/or propulsion power.

[003] A dual-fuel engine uses a low-pressure gaseous fuel such as natural gas that is mixed at relatively low pressure with intake air admitted into the engine cylinders. The air/gaseous fuel mixture that is provided to the cylinder under cer tain operating conditions is compressed and then ignited using a spark or using a compression ignition pilot fuel, such as diesel, which is injected into the air/gas eous fuel mixture present in the cylinder.

[004] A direct injection gas engine is also known as such, in which a gaseous fuel, such as liquefied natural gas (LNG), is injected into the cylinder at high pres sure while combustion in the cylinder from a diesel pilot is already underway. The direct injection gas engines operate on the gaseous fuel, and the diesel pilot pro vides ignition of the gaseous fuel.

[005] DK179056B1 discloses a fuel supply system for supplying high-pressure gas to a large two-stroke compression-ignited internal combustion engine. The engine is provided with a fuel injection system for injecting the supplied high- pressure gas into the combustion chambers of the engine. The fuel supply sys tem comprises a feed conduit connecting an outlet of a liquefied gas storage tank to the inlet of a high-pressure pump for transporting liquefied gas from the lique fied gas storage tank to the high-pressure pump, a transfer conduit connecting the outlet of the high-pressure pump to the inlet of a high-pressure vaporizer for transporting high-pressure liquefied gas from the high-pressure pump to the high- pressure vaporizer, a supply conduit connecting the outlet of the high-pressure vaporizer to an inlet of the fuel injection system of the engine for transporting high-pressure vaporized gas to the fuel injection system of the engine. The high pressure pump comprises two or more pump units. Each pump unit comprises a pump piston slidably disposed in a pump cylinder and a hydraulically powered drive piston slidably disposed in a drive cylinder with the drive piston coupled to the pump piston for driving the pump piston.

[006] US9188069 B2 discloses an engine fuel system having liquid and gase- ous fuel systems, each of which injects fuel directly into an engine cylinder. The gaseous fuel system is a direct injection gas system which comprises a liquefied gas storage, liquefied gas pump, liquefied gas evaporator and gaseous gas fuel rail connected to a fuel injector.

[007] Both of the above mention prior art documents suggest to control the op- eration of the gaseous fuel pump by control fluid which operates the high pres sure piston pump. Variable control of the high pressure pump requires a variable high pressure pump of the control fluid system. The prior art solutions have a general problem of complexity in structure of liquefied gas pump and its control lability. [008] An object of the invention is to provide direct injection gaseous fuel feed ing system for a two-stroke internal combustion piston engine in which is consid erably improved compared to the prior art solutions.

[009] An object of the invention is to provide two-stoke internal combustion pis ton engine having a direct injection gaseous fuel feeding system and method of operating a two-stroke internal combustion piston engine, which is considerably improved compared to the prior art solutions for large two-stroke internal com bustion piston engines. Disclosure of the Invention

[0010] Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention. [0011] According to an embodiment of the invention a direct injection gaseous fuel feeding system for a two-stroke internal combustion piston engine, the fuel feeding system comprising

- a fuel feed line extending from a source of liquefied gas o a fuel injector in the engine, which fuel feed line comprises at least the following - a liquefied gas high pressure pump unit

- a heat exchanger unit for evaporating the liquefied gas and heating the gaseous gas, and

- a gaseous gas accumulator between the heat exchanger unit and the fuel in jector, and - at least one gaseous gas fuel injector in fluid communication with the gaseous gas accumulator, wherein the liquefied gas high pressure pump unit comprises a reciprocating piston having a pumping part and a drive part, and a hydraulic drive assembly arranged to subject hydraulic power fluid at constant pressure from a source of hydraulic power fluid to the drive part of the reciprocating piston for driving the piston in reciprocating manner.

[0012] According to an embodiment of the invention the reciprocating piston is a double acting piston and the hydraulic drive assembly arranged to controllably subject hydraulic power fluid at constant pressure from the source of hydraulic power fluid to the drive part of the double acting piston. [0013] According to an embodiment of the invention the engine is a large two- stroke marine crosshead engine.

[0014] According to an embodiment of the invention the gaseous fuel feeding system is configured to feed fuel to one cylinder of a large two-stroke crosshead engine, the gaseous fuel feeding being connected to a common liquefied gas storage for the cylinders of the engine. [0015] Since each on the cylinders of the engine is provided with a dedicated the gaseous fuel feeding system it is possible to isolate individual cylinders from the fuel system for example for maintenance and still maintain engine running mak ing use of other cylinders of the engine and combusting liquid fuel in such a cyl- inder from which the gaseous fuel feed has been cut off.

[0016] According to an embodiment of the invention the hydraulic drive assembly arranged to controllably subject hydraulic power fluid to the drive part of the re ciprocating piston by directional valve of the hydraulic drive assembly being con trollably driven back and forth between the first position and the second position. [0017] According to an embodiment of the invention the hydraulic drive assembly of the liquefied gas high pressure pump unit comprises a 4/3-way, directional valve configured to supply pressurized power fluid alternatively to a first side or a second side of a drive part of the double-acting piston or to lock the position of the piston and stop and/or lock the position of the piston to a desired position. [0018] According to an embodiment of the invention the hydraulic drive assembly of the liquefied gas high pressure pump unit comprises a 4/2-way, directional valve configured to supply pressurized power fluid alternatively to a first side or a second side of a drive part of the double-acting piston.

[0019] According to an embodiment of the invention the hydraulic drive assembly is configured to reciprocate the piston with full strokes, each one of the strokes in creasing the pressure with a predetermined amount, until the injection pressure is at a level from which a full, single stoke would increase the pressure above a set value, and - to move the piston with partial stroke from a bottom dead center to a po sition between its extreme positions, and to stop the movement of the piston.

[0020] According to an embodiment of the invention the hydraulic drive assembly is configured to reciprocate the piston with full strokes, each one of the strokes in- creasing the pressure with a predetermined amount, until the injection pressure is at a level from which a full, single stoke would increase the pressure above a set value, and to move the piston with partial intake stroke to a position between its ex treme positions, and move the piston to a top dead center position of the piston. [0021 ] According to an embodiment of the invention the pumping part and a drive part of the liquefied gas high pressure pump unit are configured such that the maximum pressure of the liquefied gas obtainable from the pumping part with a predetermined pressure of the hydraulic power fluid corresponds to the set injec tion pressure. [0022] According to an embodiment of the invention the hydraulic drive assembly is configured to reciprocate the piston with a frequency of 0,5 - 6, preferably 3 - 6 times per second.

[0023] According to an embodiment of the invention the hydraulic drive assembly is configured to reciprocate the piston in an intermittent manner. [0024] According to an embodiment of the invention the liquefied gas high pres sure pump unit is configured to raise the pressure of the liquefied gas up to a pressure which is about 0,5 times the pressure of the hydraulic power fluid, wherein the pressure of the fluid is in the range of 12 - 15 MPa.

[0025] A two-stroke in internal combustion piston engine according to the inven- tion comprising more than one cylinders in which each cylinder is provided with a direct injection gaseous fuel feeding system according to anyone of the ap pended claims.

[0026] According to an embodiment of the invention each cylinder of the engine comprises two gaseous gas injectors and three liquid fuel injectors. [0027] Method of operating a two-stroke internal combustion piston engine com prising a plurality of cylinders, a common rail liquid fuel injection system and a direct injection gaseous fuel feeding system, the method comprising steps of run ning the engine such that at least one of the cylinders is run by injecting solely liquid fuel to the cylinder and the remaining cylinders are run by injecting both gaseous fuel and liquid fuel into each one of the remaining cylinders. [0028] The exemplary embodiments of the invention presented in this patent ap plication are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. The novel features which are considered as charac teristic of the invention are set forth in particular in the appended claims. Brief Description of Drawings

[0029] In the following, the invention will be described with reference to the ac companying exemplary, schematic drawings, in which

Figure 1 illustrates a direct injection gaseous fuel feeding system according to an embodiment of the invention, Figures 2 to 4 illustrates operational steps of high pressure pump unit and hy draulic drive assembly according to an embodiment of the invention,

Figure 5 illustrates a two stroke internal combustion engine according to still an other embodiment of the invention, and

Figures 6 to 7 illustrates operational steps in high pressure pump unit and hy- draulic drive assembly according to another embodiment of the invention.

Detailed Description of Drawings

[0030] Figure 1 depicts schematically a direct injection gaseous fuel feeding sys tem 10 for a large two-stroke internal combustion piston engine, showing a fuel feeding system 10 of a section for one cylinder 12 of the engine. The engine has at least one cylinder that forms a variable volume between a reciprocating piston, a bore, and a cylinder head or alike. The cylinder 12 of the engine is illustrated in the figure 1 only in very schematically, since generally the structure and oper ation of a large two-stroke cross head internal combustion piston engine is well known to those skilled in the art. The liquid fuel system includes a common rail with a one or several liquid fuel injectors adapted to inject liquid fuel directly into the variable volume as an ignition source. The gaseous fuel system is in flow connection with a tank which is a storage or a source of liquefied gas, a high pressure pumping unit, a drive assembly of the pump unit, and a gaseous fuel injector. In the engine the compressed gaseous fuel is ignited by separate liquid fuel injection system.

[0031] The fuel feeding system 10 comprises a source of liquefied gas that is here a liquefied gas storage tank 14 configured to store liquefied gas in cryogenic conditions. In practise the gas is stored in the tank typically at temperature -160 to -150 °C and pressure of 200 to 300 kPa. There is a fuel feed line 16 provided in the system, extending from the storage tank 14 to a fuel injector 19 in the engine 1.

[0032] The fuel feed line 16 comprises a liquefied gas high pressure pump unit 18 coupled to the fuel feed line 16. There is also a liquefied gas low pressure pump unit 20 arranged between the high pressure pump unit 18 and the storage tank 14 to the fuel feed line 16. The low pressure pump 20 is configured to raise the pressure of the liquefied gas to about 900 kPa. The low pressure pump unit acts as a transfer pump and it improves the operation of the system such that the risk of excessive pressure drop at the high pressure pump unit 18 during suction stoke can be at least minimized, preferably totally avoided. In the high pressure pump unit 18, if the liquid pressure drops below the vapor pressure, liquid boiling will occur, and cavitation and loss of prime of the pump may result. Vapor bubbles may reduce or stop the liquid flow, reduce pump efficiency, lower the mass of the compressed liquid, and possibly damage the system. All these can be avoided - or at least minimized - by the combination of the high pressure pump unit 18 and suitably dimensioned low pressure pump 20 according to the aspect of the in vention. The high pressure pump unit 18 is configured to raise the pressure of the liquefied gas from about 900 kPa up to 15 MPa.

[0033] The fuel feeding system 10 comprises further a heat exchanger unit 22 downstream the high pressure pump 18 in the gas flow direction. The heat ex changer unit 22 is arranged to the fuel feed line 16 downstream the high pressure pump unit 18. The heat exchanger unit 22 is configured to withhold the pressure of at least 15 MPa which is advantageously also a set value for the gaseous gas injection pressure. While the high pressure pump unit 18 is configured to operate with liquefied gas at cryogenic conditions the heat exchanger unit 22 evaporates the pressurized liquid gas into gaseous form and heats the gaseous gas to an elevated temperature, compared to that prevailing in the storage tank 14. Heat needed for evaporation of the liquefied gas into gaseous gas may be fully of partly obtained from the engine 1. There may also be provided an auxiliary heat source such as an electric heater or a boiler. In this connection the gas is referred to as liquefied gas and gaseous gas in order to distinguish the current phase of the gas. So, the gas is referred to as gas even if being in liquid phase or in super- critical phase.

[0034] The fuel feeding system 10 comprises further a gaseous gas accumulator 24. The accumulator is configured to store gaseous gas at temperature of about -50 to -20 °C and pressure of about 12 to 15 MPa. The accumulator is arranged between the heat exchanger unit 22 unit and the fuel injector 19. As can be seen from the figure 1 each cylinder of the engine comprises two gaseous fuel injectors 19 which are both in flow communication with the common gaseous gas accu mulator 24. The gaseous gas accumulator 24 is provided with a temperature sen sor 28 and a pressure sensor 26 the measurement signals of which is advanta geously utilized in controlling operation of the fuel feeding system 10. The gase- ous gas accumulator comprises an inlet for gaseous gas 25 into which the fuel feed line 16 is connected to, and outlets 27 for connecting the fuel feed lines 16 which connects each one of the at least two fuel injectors 19 to the accumulator 24. The gaseous gas accumulator 24 is also provided with an additional outlet 23. There is a gaseous gas return conduit 60 arranged in the system which ex- tends from the additional outlet 23 of the gaseous gas accumulator 24 to the storage tank 14. The gaseous gas return conduit 60 is provided with a start-up and safety valve 62 controlled by a 3/2-way directional valve 64. There is also a three-way valve 66 in connection with the return conduit 60, by means of which the start-up and safety valve 64 and the storage tank 14 can be selectively con- nected with each other and/or with fuel further processing. The start-up and safety valve 62 is in direct flow connection with a control oil feed conduit 106 and via the 3/2-way directional valve to a control oil return conduit 108. [0035] The liquefied gas high pressure pump unit 18 comprises a reciprocating piston 30 having a pumping part 32 and a drive part 34, and a hydraulic drive assembly 36 which arranged to controllably subject hydraulic power fluid to the drive part of the reciprocating piston 30. The pumping part 32 comprises a cylin- der 38 in which the piston 30 is arranged to reciprocate. The cylinder 38 and the piston 30 borders a pumping chamber 40 at one side of the piston, the volume of which increases and decreases according to the position of the piston 30 being smallest at the top dead center of the piston 30 and largest at the bottom dead center of the piston 30. The fuel feed line 16 connecting the pump unit 18 with the storage tank 14 is in flow communication with the pumping chamber 40. There is a first one-way valve 42 in connection with the pumping chamber allow ing the flow of the liquefied gas only into the pumping chamber 40. And, fuel feed line 16 connecting the pump unit 18 with the heat exchange 22 is in flow commu nication with the pumping chamber 40 also. And, there is also a second one-way valve 44 in connection with the pumping chamber allowing the flow of the lique fied gas to flow only out from the pumping chamber 40. The one-way valves may be integrated to the high pressure pump unit 18

[0036] The fuel feeding system is also provided with a fuel return line 16’ for returning possible leak fuel from the pump unit back to the storage tank 14. Both the fuel return line 16’ and the fuel fed line 16 is provided with a valve 13 which makes it possible to isolate a fuel feeding system of a single cylinder while main taining the other cylinders using the gaseous fuel feeding system normally. This way some of the cylinders may be operated is diesel mode by combusting liquid fuel with compression ignition and some of the cylinders may be operated with gaseous fuel with ignition by liquid fuel wherein the liquid fuel is ignited by com pression ignition and the gaseous fuel is ignited by the combustion of the liquid fuel. There is a restriction valve 15 arranged to the return line 16’ just upstream or before the tank 14 so as to maintain the pressure in the return line substantially at a level to which the low pressure pump 20 rises the pressure of the liquefied gas.

[0037] In the figure 1 the hydraulic drive assembly 36 of the liquefied gas high pressure pump unit comprises a 4/3-way, directional valve 46 configured to sup ply pressurized power fluid alternatively to a first side or a second side of a drive part double-acting piston or to lock the position of the piston at a desired location. In this connection the expression 4/3-way means that the valve has 4 ports and the number of positions is 3.

[0038] The gaseous fuel feeding system 10 comprises a control fluid system 100 for operating the hydraulic drive assembly, which preferably uses oil as its work ing medium and may therefore be also referred to as a control oil hydraulic sys tem 100. The control oil system comprises a tank 102 which is a storage or a source of hydraulic power fluid i.e. the control oil. The control fluid system 100 further comprises a high pressure pump 104 which pressurizes the control oil suitably for operating the hydraulic drive assembly 36, preferably up to 30 MPa. The control oil system comprises also a control oil feed conduit 106, which is configure to deliver the pressurized control fluid to any desired apparatus in the fuel feeding system 10, but particularly to the hydraulic drive assembly 18, as substantially constant pressure. There is also a control oil return conduit 108 in the system for returning the control oil back to the tank 102. The control oil feed conduit 106 and the control oil return conduit 108 are in flow connection with respective ports of the directional valve 46 of each cylinder of the engine. This way the piston 30 of the pump unit 18 is actively driven into both of its optional directions with the pressure of control oil in the control oil system 100. [0039] The injector is advantageously substantially such as is disclosed in the

International Publication Number WO 2017/162902 A1 particularly relating to the first fuel feeding section described therein. Also the injector comprises sealing system as is described in the publication using the term sealing fluid chamber corresponding to the sealing system, the description of which is incorporated herein with reference. The control fluid system 100 is in flow connection with each one of the injectors such that the pressurized control oil is fed to the sealing sys tem 17 of the injectors through control oil feed conduit 106 and returned to the tank via the oil return conduit 108.

[0040] In the figures 2, 3 and 4 the operation of high pressure pump unit 18 and the hydraulic drive assembly 36 of the figure 1 is described in more detailed man ner. First referring to the figure 2, the piston 30 of high pressure pump unit 18 is shown at an end of its intake stroke i.e. at its bottom dead center. That is when the piston 30 in the pumping part 32 is at its extreme position (left in the figure 2) and the pumping chamber 40 has its maximum volume. Respectively, the piston 30 in the drive part 36 is moved to its extreme position of the intake stroke. The high pressure control oil has been flown through the directional valve 46 from a pressure port 46.1 to a port 46.2 which is in flow connection with a chamber in the drive part 36 which causes the movement of the intake stroke. During the movement of the piston 30 towards its end of intake stroke position liquefied gas is taken into the chamber 40 from the fuel feed line 16 allowed by the one-way valve 42, as is shown by the arrow in the chamber 40. As is discussed earlier, since the low pressure pump 20 increases the pressure in the fuel feed line the risk of irregularities, such as cavitation due to the sucking movement of the piston can be minimized and the speed of the piston can be maximized.

[0041] The directional valve 46 of the hydraulic drive assembly 36 has four ports 46.1, 46.2, 46.3, 46.4 from which the port 46.1 is a pressure port and the port 46.3 is a tank port. The tank port 46.3 is provided with a first controllable re- striction 48. Also the conduit between the fourth port 46.4 and the drive part 34 of the piston 30 at the side behind the piston i.e. the side of increasing volume during pumping stroke action i.e. compression stroke, is provided with a second controllable restriction 50. In the figure 2 directional valve 46 is set at a first posi tion in which the pressure port 46.1 is in connection with a side of a drive side piston which urges the piston 30 to the direction where the pumping chamber 40 has its maximum volume. The returning flow of control oil flows through the first constriction 48 and the second construction 50 by means of the speed of the piston 30 is maintained low enough to avoid any cavitation in the pumping cham ber 40. [0042] In the figure 3 there is shown a second position of the directional valve 46 of the hydraulic drive assembly 36. It is in the at-rest position, between the first position shown in the figure 2 and the third position shown in the figure 3, where all ports are blocked and the position of the piston 30 is locked in the pump unit 18. [0043] In the figure 4 directional valve 46 is set at a third position. In the third position the pressure port 46.1 is in connection with both sides of a drive side 34 of the piston 30 Now the effective areas causing the resultant force to the piston due to the pressure, at both sides of the piston in the drive side are such that the ratio of the areas is about 1:3 - 1:5, preferably about 1:4. This way the force admitted to the piston during to compression stoke is about 4 times the force admitted to the piston during the intake stroke which urges the piston 30 to the direction where the pumping chamber 40 has its minimum volume, i.e. to towards the top dead center position of the piston 30. When the pump unit 18 is controlled to be operating, the directional valve of the hydraulic drive assembly 36 is con trolled back and forth between the first position (figure 2) and the third position (figure 4) wherein the same power fluid in the control fluid system, and thus same and constant pressure, is used for obtaining the reciprocating movement of the piston 30 during the pumping sequence. The pressurized control oil supply is selectively in flow connection from the pressure port 46.1 with both sides of the piston 30 at the drive part 34. This way, the hydraulic drive assembly 36 is con figured control the pump unit 18 to perform a pumping sequence, during which the piston 30 is arranged to reciprocate in the pump unit 18. This is accomplished by activating the directional valve 46 to move between the first and the third po sitions.

[0044] More advantageously, the hydraulic drive assembly 36 is configured to first reciprocate the piston 30 with full strokes between top dead center and bot tom dead center, each one of the strokes increasing the pressure of the liquefied gas with a predetermined amount, step by step or stroke by stroke. This is called full stroke pumping sequence. That is practised until the liquefied gas pressure, preferably corresponding to the injection pressure, is at a level from which a full single stroke of the piston 30 would increase the pressure above a set value. Typical state of the liquefied gas after the high pressure pump unit 18 is such that the pressure is 12 MPa - 15 MPa and temperature being -145 to -135 °C.

[0045] According to an embodiment of the invention at this stage of the pumping sequence, i.e. after the last full stroke of the piston 30, the hydraulic drive assem bly 36 is configured to return the piston 30 from its top dead center to the bottom dead center and move the piston only for a distance which results the next partial pumping stroke causing the pressure of the liquefied gas to rise up to the set target pressure i.e. a partial stroke of the piston 30 to a position between its ex treme positions is performed. Then the movement of the piston is stopped, until the next pumping sequence is initiated. 4/3-way directional valve can be a pro portional control valve with position feedback, which can be used for determining the stopping position for the piston.

[0046] The length of the partial stroke can be determined though calculation based on measured variables (liquefied gas pressure, temperature, piston and chamber dimensions, piston position) made available from the system. For ex ample the piston may be moved until a set target pressure is obtained and veri fied by a pressure measurement, or the piston may be moved to a position at which the pressure is calculated to the target pressure, verified by a position sen- sor.

[0047] According to another embodiment of the invention at this stage of the pumping sequence, i.e. after the last full stroke of the piston 30, the hydraulic drive assembly 36 is configured to return the piston 30 from its top dead center towards the bottom dead center only for a distance which results the next partial pumping stroke back to the top dead center causing the pressure of the liquefied gas to rise up to the set target pressure i.e. a partial stroke of the piston 30 from a position between its extreme positions back to the top dead center position is performed. Then the movement of the piston is stopped, until the next pumping sequence is initiated. [0048] The length of the partial stroke can be determined though calculation based on measured variables (liquefied gas pressure, temperature, piston and chamber dimensions, piston position) made available from the system.

[0049] An electronic control unit ECU, i.e. a control computer, of the fuel feeding system comprises a computer program, which when executed controls the oper- ation of the fuel feeding system, particularly the high pressure pump unit 18 as described above. The ECU and the computer program are provided for configur ing the system and/or its various part to operate in the manner described heren.

[0050] During the full stroke pumping sequence the position of the directional valve is changed and the piston 30 is advantageously arranged to reciprocate with a frequency of 3 - 6 times per second between its top dead center and bottom dead center. The hydraulic drive assembly is configured to reciprocate the piston in an intermittent manner such that its operation is based on the pres sure measurement 28 in the accumulator 24 and independently from the activa tion of the injection valve 19. Advantageously the output of the pump unit 18 during one pumping sequence substantially corresponds to injection amount of the fuel for one engine cycle through the injectors 19 in the cylinder.

[0051] Referring now back to the figure 1, the gaseous gas pressure in the ac cumulator 24 for the gaseous fuel injectors 19 in one cylinder of the engine is maintained at desired pressure, that is advantageously 12 MPa - 15 MPa, which can also be denoted as injection pressure. The pressure in the accumulator 24 is measured by the sensor 28, which value is used in the control procedure for controlling operation of the liquefied gas high pressure pump unit 18 as is de scribed above. A desired minimum and maximum injection pressure are set. The measured pressure in the accumulator 24 is compared with the minimum and maximum injection pressure. If the measured pressure is lower than the minimum target pressure, the pump unit 18 is started and operated until the measured pressure in the accumulator is at least the maximum injection pressure. This way the injection pressure is controlled by means of the high pressure pump unit 18 and the pressure measurement 28 in accumulator 24.

[0052] Further the gaseous gas in the accumulator is maintained at desired tem- perature that is -50 to -20 °C. The temperature in the accumulator is measured by the sensor 26, which value is used in the control procedure for controlling operation of the heat exchanger unit 22. A desired minimum and maximum tem perature is set. The measured temperature in the accumulator 24 is compared with the minimum and maximum set temperatures. If the measured temperature is lower than the minimum target temperature, the heat exchanger unit heating power is maintained as it is or increase until the measured temperature in the accumulator is at least the maximum set temperature. This way the gaseous fuel temperature is controlled by means of the heat exchanger unit 22 and the tem perature measurement 26 in accumulator 24. [0053] Volume of the accumulator 24 depends on the needed injection amount and the total volume of the fuel feeding system between the high pressure pump unit 18 and the gas injectors 19. The volume of the accumulator is advanta geously 3 -10 times to the volume of injected fuel by the two injectors of one cylinder during full load operation. The volume is typically 2.5 - 10 litres.

[0054] Figure 5 discloses schematically a cylinder cover 200 of a large two- stroke crosshead piston engine 1 seen from inside the cylinder 204. The large 2- stroke engine, to which the gaseous fuel feeding system according to the inven tion is specifically intended, is a low speed, crosshead engine, having a cylinder bore diameter more than about 50 cm up to 1m and stroke length of about 2,5 meters. There is shows an exhaust valve 202 in the center of each cylinder and injectors 19, 20T of the engine 1 in the cylinders. According to an embodiment of the invention the two-stroke internal combustion piston engine comprises at least two cylinders in which each cylinder is provided with a direct injection gas eous fuel feeding system having two injectors 19 in the cylinder and additionally three liquid fuel injectors 201 coupled to a common rail fuel injection system 206. It is known as such for a skilled person in the art of large two-stroke engines that the number of liquid fuel injectors may vary. The liquid fuel injectors 201 and the common rail liquid fuel injection system 206 are configured to inject for example heavy fuel oil, light fuel oil and/or marine fuel oil. This way any one of the cylinders of the two-stroke engine can be operated in dual fuel mode with gaseous fuel as its main fuel, ignited with the liquid fuel, but also in diesel mode with liquid fuel only as diesel engine if so desired. In the dual fuel mode the cylinder is fuelled by combination of gaseous gas and liquid fuel. Advantageously, according to the invention the engine is run at least in special situations such that at least one of the cylinders is run by injecting solely liquid fuel to the cylinder and the remaining cylinders are run by injecting both gaseous fuel and liquid fuel into each one of the remaining cylinders. Advantageously in the at least one of the cylinders which run by injecting solely liquid fuel to the cylinder, the direct injection gaseous fuel feeding system is serviced during the engine is running.

[0055] In the figures 6 and 7 the operation of high pressure pump unit 18 and the hydraulic drive assembly 36 according to another embodiment of the inven tion is described in more detailed manner. The high pressure pump unit 18 and the hydraulic drive assembly 36 in the figure 6 and 7 are intended for use in connection with a gaseous fuel feeding system in the figure 1 replacing the high pressure pump unit 18 and the hydraulic drive assembly 36 shown therein. First referring to the figure 6, the piston 30 of high pressure pump unit 18 is shown at an end of its intake stroke i.e. at its bottom dead center position. That is when the piston 30 in the pumping part 32 is at its extreme position (left in the figure 6) and the pumping chamber 40 has its maximum volume. Respectively the piston 30 in the drive part 36 is moved to its extreme position of the intake stroke. The high pressure control oil is flown through the directional valve 46 from a pressure port 46.1 to a port 46.2 which is in flow connection with a chamber in the drive part 36 which causes the movement of the intake stroke. During the movement of the piston 30 towards its end of intake stroke position liquefied gas is taken into the chamber 40 from the fuel feed line 16 allowed by the one-way valve 42, as is shown by the arrows. As is discussed earlier, since the low pressure pump 20 increases the pressure in the fuel feed line the risk of irregularities due to the sucking movement of the piston can be minimized and the speed of the piston can be maximized.

[0056] The directional valve 46 of the hydraulic drive assembly 36 is a 4/2-way directional valve and it has four ports 46.1, 46.2, 46.3, 46.4 from which the port 46.1 is a pressure port and the port 46.3 is a tank port. The tank port 46.3 is provided with a first controllable restriction 48. Also the conduit between the fourth port 46.4 and the drive part 34 of the piston 30 at the side behind the piston i.e. the side of increasing volume during pumping stroke action, is provided with a second controllable restriction 50. In the figure 6 directional valve 46 is set at a first position in which the pressure port 46.1 is in connection with a side of a drive side piston which urges the piston 30 to the direction where the pumping chamber 40 has its maximum volume. The returning flow of control oil flows the fourth port

46.4 and the tank port 46.3 and therefore, as is seen in the figure, through the first constriction 48 and the second construction 50 by means of which con strictions the speed of the piston 30 is maintained low enough to avoid any cavi tation is the pumping chamber 40. [0057] In the figure 7 the 4/2 way directional valve 46 is set at a second position.

In the second position the pressure port 46.1 is in connection with the ports 46.2 and 46.4 and this way it is also in connection with both sides of a drive side piston. Due to the differences in the effective piston areas between the piston sides at the drive side of the piston the pressure of the control fluid urges the piston 30 to the direction where the pumping chamber 40 has its minimum volume. The ef fective areas causing the resultant force to the piston are such that the ratio of the areas is about 1:3 - 1:5, preferably about 1:4. [0058] When the pump unit 18 according to the figure 6 and 7 is controlled to be operating, the directional valve of the hydraulic drive assembly 36 is controlled back and forth between the first position (figure 6) and the third position (figure 67 wherein the same power fluid and fluid pressure in the control fluid system is used for obtaining the reciprocating movement of the piston 30 to both directions. Now, the hydraulic drive assembly 36 is configured control the pump unit 18 to perform a pumping sequence during which the piston 30 is arranged to recipro cate in the pump unit 18. This is accomplished be activating the directional valve 46 to move between the first and the third positions. Typical state of the liquefied gas after the high pressure pump unit 18 is such that the pressure is 12 MPa - 15 MPa and temperature being -145 to -135 °C.

[0059] According to the embodiment shown in the figures 6 and 7, the hydraulic drive assembly 36 is advantageously configured to reciprocate the piston 30 with full strokes, each one of the strokes increasing the pressure of the liquefied gas with a predetermined amount, step by step or stroke by stroke. That is practised until the liquefied gas pressure, preferably the injection pressure, so high that the pump cease to move. The pumping part 32 and a drive part 34 of the liquefied gas high pressure pump unit 18 are configured such that the maximum pressure of the liquefied gas obtainable (preferably or for example 15 MPa) from the pump ing part with a predetermined pressure (preferably or for example 900 kPa) of the hydraulic power fluid corresponds to the set injection pressure (preferably or for example 15 MPa). In other words the pump unit 18 is configured to raise the pressure of the liquefied gas up to a pressure which is about 0,5 times the pres sure of the control oil. When the reciprocating movement of the piston is detected to be ceased the pumping sequence is stopped. After that there are at least two optional procedures to follow. Firstly, it is possible to leave control fluid pressure on, to push the piston 30 to press the liquefied gas, in which case the piston 30 may move to its top dead center position as pressure in the accumulator 24 de creases. Secondly, it is possible to commence the intake stroke of the piston 30 immediately.

[0060] An electronic control unit ECU, i.e. a control computer, of the fuel feeding system comprises a computer program, which when executed controls the oper ation of the fuel feeding system, particularly the high pressure pump unit 18 as described above.

[0061] During the pumping sequence the position of the directional valve is changed and the piston 30 is advantageously arranged to reciprocate with a fre- quency of 3 - 6 times per second. The hydraulic drive assembly is configured to reciprocate the piston in an intermittent manner such that its operation is based on the pressure measurement 28 in the accumulator 24 and independently from the activation of the injection valve 19. Advantageously the output of the pump unit 18 during one pumping sequence substantially corresponds to injection amount of the fuel for one engine cycle through the two injectors 19.

[0062] According to an embodiment of the invention, which can be applied to the both set of figures 2-4 and 6-7 and the operations relating to the figures, after the last full stroke of the piston 30 the hydraulic drive assembly 36 is configured to return the piston 30 from its top dead center only for a distance (partial intake stroke) which causes the next partial pumping stroke up to the top dead center to result in the pressure of the liquefied gas to rise up to the set target pressure. Then the movement of the piston is stopped until the next pumping sequence is initiated. Before next pumping sequence, and after each pumping sequence the piston 30 is advantageously returned to its top dead center position. [0063] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred em bodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the in- vention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodi ment when such combination is technically feasible. Numbered list of embodiments

Following embodiments describe alternative advantageous implementations of the invention. 1. A direct injection gaseous fuel feeding system for a two-stroke internal combustion piston engine, the fuel feeding system comprising

- a fuel feed line extending from a liquefied gas source to a fuel injector in the engine, which fuel feed line comprises at least the following

- a liquefied gas high pressure pump unit - a heat exchanger unit for evaporating the liquefied gas and heating the gaseous gas, and

- a gaseous gas accumulator between the heat exchanger unit and the fuel in jector, and

- at least one gaseous gas fuel injector in fluid communication with the gaseous gas accumulator, wherein the liquefied gas high pressure pump unit comprises a reciprocating piston having a pumping part and a drive part, and a hydraulic drive assembly arranged to subject hydraulic power fluid at constant pressure from a source of hydraulic power fluid to the drive part of the reciprocating piston for driving the piston in reciprocating manner. 2. A direct injection gaseous fuel feeding system according to embodiment

1 , wherein the hydraulic drive assembly arranged to controllably subject hydraulic power fluid at constant pressure from the source of hydraulic power fluid to the drive part of the reciprocating piston.

3. A direct injection gaseous fuel feeding system according to embodiment 1 or 2, wherein the reciprocating piston is a double-acting piston.

4. A direct injection gaseous fuel feeding system according to embodiment 1 or 2, wherein the hydraulic drive assembly arranged to controllably subject hy draulic power fluid to the drive part of the reciprocating piston by a directional valve of the hydraulic drive assembly being controllably driven back and forth between the first position and the second position. 5. A direct injection gaseous fuel feeding system according to embodiment 1 or 2, wherein the hydraulic drive assembly of the liquefied gas high pressure pump unit comprises a 4/3-way, directional valve configured to supply pressur ized power fluid alternatively to a first side or a second side of a drive part of the double-acting piston and stop and/or lock the position of the piston to a desired position.

6. A direct injection gaseous fuel feeding system according to embodiment 1 or 2, wherein the hydraulic drive assembly of the liquefied gas high pressure pump unit comprises a 4/2-way, directional valve configured to supply pressur- ized power fluid alternatively to a first side or a second side of the drive part of the reciprocating piston.

7. A direct injection gaseous fuel feeding system according to embodiment 1 or 2, wherein the gaseous gas accumulator comprises an inlet for gaseous gas and outlets for each one of the at least one fuel injector. 8. A direct injection gaseous fuel feeding system according to embodiment

1 or 2, wherein the fuel feeding system comprises at least of two fuel injectors.

9. A direct injection gaseous fuel feeding system according to embodiment 5, wherein the hydraulic drive assembly is configured to reciprocate the piston with full strokes, each one of the strokes increas ing the pressure with a predetermined amount, until the injection pressure is at a level from which a full, single stoke would increase the pressure above a set value, and to move the piston with partial stroke from a bottom dead center to a po sition between its extreme positions, and to stop the movement of the piston.

10. A direct injection gaseous fuel feeding system according to embodiment 5 or 6, wherein the hydraulic drive assembly is configured to reciprocate the piston with full strokes, each one of the strokes increas ing the pressure with a predetermined amount, until the injection pressure is at a level from which a full, single stoke would increase the pressure above a set value, and to move the piston with partial intake stroke to a position between its ex treme positions, and move the piston to its top dead center position.

11. A direct injection gaseous fuel feeding system according to embodiment 6, wherein the pumping part and a drive part of the liquefied gas high pressure pump unit are configured such that the maximum pressure of the liquefied gas obtainable from the pumping part with a predetermined pressure of the hydraulic power fluid corresponds to the set injection pressure.

12. A direct injection gaseous fuel feeding system according to embodiment 5 or 6, wherein the hydraulic drive assembly is configured to reciprocate the pis ton with a frequency of 0,5 - 6 times per second.

13. A direct injection gaseous fuel feeding system according to embodiment 11, wherein the hydraulic drive assembly is configured to reciprocate the piston with a frequency of 3 - 6 times per second.

14. A direct injection gaseous fuel feeding system according to embodiment 12 or 13, wherein the hydraulic drive assembly is configured to reciprocate the piston in an intermittent manner.

15. A direct injection gaseous fuel feeding system according to embodiment 11, wherein the hydraulic drive assembly is configured to reciprocate the piston first with full strokes and after the full strokes with one partial stroke until a pre determined injection pressure is reached in the gaseous gas accumulator. 16. A direct injection gaseous fuel feeding system according to embodiment

5 or 6, wherein the hydraulic drive assembly is configured to operate the pump unit with a full stroke pumping and subsequently with one partial stroke of the piston to a position between the extreme positions of the piston.

17. A direct injection gaseous fuel feeding system according to embodiment 5 or 6, wherein the hydraulic drive assembly is configured to operate the pump unit with a full stroke pumping sequence and after the last full stroke of the piston (30) the hydraulic drive assembly (36) is configured to return the piston (30) from its top dead center only for a distance which causes the next partial pumping stroke up to the top dead center to result in the pressure of the liquefied gas to rise up to the set target pressure.

18. A direct injection gaseous fuel feeding system according to embodiment 110, wherein the liquefied gas high pressure pump unit is configured to raise the pressure of the liquefied gas up to a pressure which is about 0,5 times the pres sure of the hydraulic power fluid, when the pressure of the fluid is in a range of 12 -15 MPa.

19. A direct injection gaseous fuel feeding system according to embodiment 1, wherein the liquefied gas high pressure pump unit is configured to raise the pressure of the liquefied gas up to a pressure which is about 0,5 times the pres sure of the hydraulic power fluid wherein the pressure of the fluid is in a range of 12 -15 MPa.

20. A direct injection gaseous fuel feeding system according to embodiment 2 or 19, wherein the fuel feeding system comprises a high pressure pump for the hydraulic power fluid pressure configured to deliver pressurized control fluid at substantially constant pressure.

21. A direct injection gaseous fuel feeding system according to anyone of the preceding embodiments, wherein the fuel feeding system comprises at least two fuel injectors for each cylinder of the two-stoke internal combustion piston engine. 22. A direct injection gaseous fuel feeding system according to anyone of the preceding embodiments 1-19 wherein the fuel feed line comprises a liquefied gas high pressure pump unit coupled to the fuel feed line and a liquefied gas low pressure pump unit arranged between the high pressure pump unit and a storage tank . 23. A direct injection gaseous fuel feeding system according to anyone of the preceding embodiments, wherein the fuel feeding system is provided with fuel feed line and fuel return line which both are provided with a closing valve.

24. A direct injection gaseous fuel feeding system according to embodiments 22 and 18 wherein the valve in the fuel feed line is arranged between the low pressure pump and the high pressure pump unit. 25. A direct injection gaseous fuel feeding system according to embodiment 9 or 14 wherein the hydraulic drive assembly is configured to move the piston with partial stoke such that the partial stroke starts after previous full stroke at a top dead center position of the piston, and the piston is returned by partial intake stroke and pumping stroke back to the top dead center position.

26. A two-stroke in internal combustion piston engine comprising more than one cylinders wherein each cylinder of the engine is provided with a direct injec tion gaseous fuel feeding system according to anyone of the preceding embodi ments having a common liquefied gas storage tank. 27. A two-stroke internal combustion piston engine according to the embodi ment 24, wherein each cylinder of the engine comprises two gaseous gas injec tors and three liquid fuel injectors.

28. A two-stroke crosshead internal combustion piston engine according to the embodiments 26 or 27. 29. A two-stroke crosshead internal combustion piston engine according to the embodiments 26 or 27 comprising a control computer comprising a computer program, which when executed controls the operation of the fuel feeding system in a manner as recited in the claims 30-31.

30. Method of operating a two-stroke internal combustion piston engine comprising a plurality of cylinders, a common rail liquid fuel injection system and the direct injection gaseous fuel feeding system according to anyone of the pre ceding embodiments 1 to 25, the method comprising steps of running the engine such that at least one of the cylinders is run by injecting solely liquid fuel to the cylinder and the remaining cylinders are run by injecting both gaseous fuel and liquid fuel into each one of the remaining cylinders.

31. Method of operating a two-stroke internal combustion piston engine according to the embodiment 30 comprising subjecting hydraulic power fluid at constant pressure from a source of hydraulic power fluid to the drive part of the reciprocating piston and driving the piston in reciprocating manner and operating the pump unit with a full stroke pumping sequence and after the last full stroke of the piston returning the piston from its top dead center only for a distance which causes the next partial pumping stroke up to the top dead center to result in the pressure of the liquefied gas to rise up to the set target pressure.

32. Method of operating a two-stroke internal combustion piston engine according to the embodiment 30 wherein in the at least one of the cylinders which run by injecting solely liquid fuel to the cylinder, the direct injection gaseous fuel feeding system is serviced.

Claims

1. A direct injection gaseous fuel feeding system (10) for a two-stroke inter nal combustion piston engine (1), the fuel feeding system (10) comprising - a fuel feed line (16) extending from a source of liquefied gas (14) to a fuel in jector (19) in the engine (1), which fuel feed line (16) comprises at least the fol lowing

- a liquefied gas high pressure pump unit (18),

- a heat exchanger unit (22) for evaporating the liquefied gas and heating the gaseous gas, and

- a gaseous gas accumulator (24) the heat exchanger unit (22) and the fuel injector (19), and

- at least one gaseous gas fuel injector (19) in fluid communication with the gas eous gas accumulator (24), characterized in that the liquefied gas high pressure pump unit (18) comprises a reciprocating piston (30) having a pumping part (32) and a drive part (34), and a hydraulic drive assembly (36) arranged to subject hydraulic power fluid at constant pressure from a source of hydraulic power fluid (102) to the drive part (34) of the reciprocating piston (30) for driving the piston in reciprocating manner. 2. A direct injection gaseous fuel feeding system (10) according to claim 2, characterized in that the hydraulic drive assembly (36) arranged to controllably subject hydraulic power fluid at constant pressure from the source of hydraulic power fluid (102) to the drive part (34) of the double acting, reciprocating piston.

3. A direct injection gaseous fuel feeding system (10) according to claim 1 or 2, characterized in that the hydraulic drive assembly (36) arranged to control lably subject hydraulic power fluid to the drive part (34) of the reciprocating piston (30) by a directional valve (46) of the hydraulic drive assembly (36) being con trollably driven back and forth between the first position and the second position.

4. A direct injection gaseous fuel feeding system (10) according to claim 1 or 2, characterized in that the hydraulic drive assembly (36) of the liquefied gas high pressure pump unit (18) comprises a 4/3-way, directional valve (46) config ured to supply pressurized power fluid alternatively to a first side or a second side of a drive part (34) of the double-acting piston, and stop and/or lock the position of the piston (30) to a desired position.

5. A direct injection gaseous fuel feeding system (10) according to claim 1 or 2, characterized in that the hydraulic drive assembly (36) of the liquefied gas high pressure pump unit (18) comprises a 4/2 -way, directional valve configured to supply pressurized power fluid alternatively to a first side or a second side of a drive part (34) of the reciprocating piston (30).

6. A direct injection gaseous fuel feeding system (10) according to claim 4 or 5, characterized in that the hydraulic drive assembly (36) is configured - to reciprocate the piston (3) with full strokes, each one of the strokes increasing the pressure with a predetermined amount, until the injection pressure is at a level from which a full, single stoke would increase the pressure above a set value, and to move the piston (30) with partial intake stroke to a position between its extreme positions, and move the piston (30) to its top dead center position.

7. A direct injection gaseous fuel feeding system (10) according to claim 5, characterized in that the pumping part (32) and a drive part (34) of the liquefied gas high pressure pump unit (18) are configured such that the maximum pres sure of the liquefied gas obtainable from the pumping part (32) with a predeter- mined pressure of the hydraulic power fluid corresponds to the set injection pres sure.

8. A direct injection gaseous fuel feeding system (10) according to claim 4 or 5 or 6, characterized in that the hydraulic drive assembly (36) is configured to reciprocate the piston with a frequency of 3 - 5 times per second.

9. A direct injection gaseous fuel feeding system (10) according to claim8, characterized in that the hydraulic drive assembly (36) is configured to recipro cate the piston in an intermittent manner.

10. A direct injection gaseous fuel feeding system (10) according to claim 1, characterized in that the liquefied gas high pressure pump unit (18) is configured to raise the pressure of the liquefied gas up to a pressure which is about 0,5 times the pressure of the hydraulic power fluid.

11. A direct injection gaseous fuel feeding system (10) according to claim 1, characterized in that the fuel feeding system (10) comprises at least two gase ous gas fuel injectors (19) for each cylinder of the two-stoke internal combustion piston engine (1).

12. A direct injection gaseous fuel feeding system (10) according to claim 1, characterized in that the fuel feed line (16) comprises a liquefied gas high pres- sure pump unit (18) coupled to the fuel feed line and a liquefied gas low pressure pump unit (20) arranged between the high pressure pump unit (18) and a storage tank (14).

13. A two-stroke in internal combustion piston engine (1) comprising more than one cylinders in which each cylinder is provided with a direct injection gas- eous fuel feeding system (10) according to anyone of the preceding claims.

14. A two-stroke internal combustion piston engine (1) according to claim 13, characterized in that each cylinder of the engine (1) comprises two gaseous gas injectors and two liquid fuel injectors (19).

15. Method of operating a two-stroke internal combustion piston engine (1) comprising a plurality of cylinders (204), a common rail liquid fuel injection system

(206) and a direct injection gaseous fuel feeding system (10) according to any one of the preceding claims, the method comprising steps of running the engine such that at least one of the cylinders (204) is run by injecting solely liquid fuel to the cylinder and the remaining cylinders (204) are run by injecting both gaseous fuel and liquid fuel into each one of the remaining cylinders.